- Big Eater: This applies to Nudge the most, but all of the Flock has a huge appetite. Justified in that flying notably takes a ton of energy and they have very fast metabolisms and rates of healing.
- LEGO Genetics: The end result of a bunch of mad scientists combining human and bird DNA together.
- Winged Humanoid: All of them have wings.
- Younger Than They Look: Thanks to genetic experimentation, all of them look older than they actually are.
- Secret Project Refugee Family: They were originally victims of The Schools constant torture and experimentation before escaping. They all see each other as one big family.
- Anti-Hero: Max is a genuinely good person who does strive to do what's right, but she's also not afraid to get her hands dirty, and doesn't usually seem very interested in talking things out with her enemies.
- Deadpan Snarker: She's a natural-born snarker, and it shows in her first person narration.
- Hair of Gold, Heart of Gold: Downplayed; she has blonde hair and, while a little rough around the edges, is generally nice.
- The Leader: Of the Flock.
- Lethal Chef: Everyone shudders at the thought of tasting Max's cooking. In fact, one of the ways the Flock knows Max II is fake is because she offers to cook.
- Promoted to Parent: With Jeb out of their lives for about two years, Max grew into her role as a leader since she was the oldest. Deconstructed in that she's clearly stressed with an excessive amount of pressure on her to live up to the parental role her Flock needs while also lacking healthy examples of how to be a proper leader (save Dr. Martinez).
- Super Speed: Her primary power, clocking in at 250 miles an hour.
- Team Mom: She's shown to regularly take care of the younger members of her flock, such as brushing Angels hair, and has a very protective attitude towards them.
- Trademark Favorite Food: Dr Martinez's chocolate chip cookies.
- Ambiguously Brown: His physical description implies that he might be of Asian descent.
- Brains and Brawn: He is the Brains to Max's Brawn. Neither are stupid exactly, however, Max is the more physically aggressive of the two while Fang encourages her to think things through.
- Dark Is Not Evil: His wings are black and he dresses primarily in black, but he's one of the good guys.
- Deadpan Snarker: Probably one of the snarkiest characters in the series, aside from Max herself.
- Invisibility: One of his mutations.
- Jerk with a Heart of Gold: He's aloof and snarky, but he does care for the Flock.
- Personality Powers: The quiet, reserved character gets invisibility powers.
- The Quiet One: Said to be the most reserved of the Flock, though at times it comes off as an Informed Attribute.
- The Stoic: He rarely shares his emotions openly.
- Tall, Dark, and Snarky: Fang is very distant and prone to being a Deadpan Snarker.
- Birds of a Feather: Puns aside, Iggy and Gazzy are this. Both have a love for explosives and fart jokes.
- Demolitions Expert: He's the resident bomb expert.
- Disabled Snarker: Being blind doesn't stop him from snarking with the best of them.
- Eye Scream: His eyes were permanently blinded after being experimented on.
- Fiery Redhead: Not in the usual sense. His hair is very close to a shade of red and Iggy enjoys making bombs in his free time.
- Handicapped Badass: Being blind doesn't stop him from kicking ass with the rest of the flock.
- Manchild: Out of everyone in the Flock, he's the closest with Gazzy, and can be as immature as him at times.
- Stepford Smiler: It's demonstrated repeatedly that underneath his joking personality, he's a sad young man who just wants his sight back.
- Supreme Chef: He's the team chef, despite being blind.
- Big Eater: The rest of the Flock is this, but Nudge is even more excessive in this area.
- Genki Girl: An excitable preteen girl, especially given the dire situation.
- Hollywood Hacking: She develops these skills in the first book, which prove useful throughout the series.
- I Just Want to Be Normal: Out of all the members of the Flock, she desires being a normal kid the most.
- Magnetism Manipulation: She gains this power in the fourth book.
- Motor Mouth: Especially when she becomes passionate about what she's talking about.
- Psychometry: Her primary mutation; she can see into the pasts of objects, which is especially useful whenever the Flock needs to hack into a computer.
- Skunk Stripe: She has a platinum blonde streak in her otherwise black hair.
- Token Minority: She's black and is the only member of the Flock explicitly confirmed to be non-Caucasian.
- Big Brother Instinct: Despite being very young, he has this towards Angel, his biological little sister.
- Demolitions Expert: Like Iggy, Gazzy is skilled in making bombs.
- Gasshole: While originally attributed to a digestion issue, it's revealed in the fourth book that Gazzy's flatulence is actually his superpower.
- Toilet Humor: Especially prone to this. His name is The Gasman, for goodness' sake. Justified in that he's an eight-year-old.
- Voice Changeling: He can change his voice to mimic anyone he wants.
- Blue-and-Orange Morality: One interpretation of her behavior is that, due to her age, life circumstances, and winning the Superpower Lottery, Angel's mental development and general morality have been heavily skewed and she can't fully comprehend when she's doing something horrible.
- The Baby of the Bunch: She's the youngest of the Flock.
- Creepy Child: She looks (and often acts) like an innocent girl, which makes her morally ambiguous tendencies even more disturbing.
- Mind Control: One of her primary powers.
- Seers: She develops future sight late into the series.
- Superpower Lottery: Described as having hit "the genetic jackpot" in terms of powers.
- Telepathy: Her other primary power.
- Token Evil Teammate: Despite being the youngest, she is this, as she rivals Jeb in her constant ping-ponging of morality.
- Voluntary Shapeshifting: A power that was only used once and never brought up again in the series.
- Wise Beyond Their Years: She often acts older than her age.
- Abusive Parents: A strange variation. By the end of the series, its stated that Jeb is definitively working for the School, meaning that he was likely aiding in the orchestration of all of the Flock's suffering. He also is quite dismissive of most of the Flock, sans Max, and is willing to sacrifice them when they don't prove useful. And once they call him out on his bull, he pretends that he was actually intending something else.
- Max is an even more excessive case, despite being Jeb's favorite. He abandoned the Flock and forced her to take the role of parent that they needed, constantly pushes her to fulfill her purpose of saving the world, and is willing to sacrifice other people for her sake, which puts an ungodly amount of pressure on her.
- Ari has this even worse, as he was blatantly emotionally neglected because of Jeb's interest in the Flock and he ultimately ditches Ari to raise the Flock in their mansion instead. While being turned into an Eraser was not Jeb's fault (The School did it while Jeb was with the Flock), rather than not involve his child in the School's agenda, he begins using Ari as muscle to get the Flock. When Ari dies, Jeb mourns, but he certainly doesn't feel bad enough to not replace him later.
- Happily Adopted: Jeb originally took in the Flock when they escaped from the School and is the closest thing to a parent that they have. Let's say that this is no longer the case by the end of the story.
- Luke, I Am Your Father: Worked with the scientists that experimented on Max and the others, and is revealed to be her father.
- Our Werewolves Are Different: Shapeshifting werewolves that have wings.
- Anti-Villain: He is absolutely ruthless towards the Flock, but he's also a severely neglected seven-year-old in the body of a monster taking out his pain on the people who "took" his father's attention.
- Creepy Child: Ari is a seven year old who openly fantasizes about kidnapping Max and cutting off her wings to stop her from ever leaving him.
- HeelFace Turn: Ari in the third book fully acknowledges that his behavior towards Max was unwarranted and that Jeb was to blame for their animosity as the adult.
- Redemption Equals Death: When he and Max start getting along and he's let go of all his emotional baggage around her, his expiration date hits and he perishes.
- Younger Than They Look: He's a child but has the body of a fully grown werewolf man.
- Kindly Vet: She takes good care of animals and is revealed to be Maxs mother.
- Satellite Love Interest: To Iggy.
- Acrophobic Bird: The humanoid version. She had to hide her wings pretty much her whole life, so she knows alleys and sewers much better than wind currents.
- Book Dumb: Comprehensive memory of her hometown's geography. Does not know what an ocean is.
- Cool Pet: The literal hawk Ridley, who is perhaps the only remnant of Hawk's old life. (She was a gift from Hawk's parents.)
- Daddy Had a Good Reason for Abandoning You: She is, quite naturally, unwilling to believe anything her parents say upon meeting them again. She eventually accepts that they are fundamentally good people, but that doesn't instantly heal 10 years of angst and paranoia.
- It should be noted that the Flock didn't "abandon" her, they left her with a trusted friend who died unexpectedly from a Diabolus ex Machina. Still sucks, but they tried a lot harder than most examples of the trope.
- Do Not Call Me "Paul": She is uncomfortable when the Flock call her Phoenix, because that name is from a childhood she has no memory of and usually associated with a lot of sentimentality she has no idea how to react to.
- Good Is Not Nice: She cuts a deal with the Flock: if she helps them rescue Fang, they help her rescue her 'kids'. After the first part happens, they learn where another missing member of theirs is being held, and that she'll be executed very soon...and then Hawk points a gun at Fang and reminds the Flock of their promise. It's not that she wants anyone hurt, or doesn't empathize, or underestimates the urgency of the situation. But her people are dying, just as surely as their friend is, and she must do whatever she can to save them. (The Flock are not pleased, but they understand, because they would do the same thing.)
- Heaven Above: She takes to worshiping the sun, once she's seen it uncovered by smog for the first time. Overlaps with Parody Religion.[T]his new sun was the most beautiful, most powerful thing Id ever seen.
- The Navigator: Her main role in the Flock. They can all fight and fly better than her, but her knowledge of the City of the Dead's landmarks and secret passages comes in handy several times.
- Secret Legacy: She had seen art of other winged people, but thought they were just mythical beings.
- Sins of Our Fathers: She hates Pietro because of his dad, regardless of anything Pietro actually does.
- Something Only They Would Say: She refuses to believe Fang is her father until he tells her that he knows she has wings (which she keeps secret from almost everyone) and describes their coloration in detail.
- Spin-Offspring: Complete with a Patchwork Kids appearance. The fandom is pretty split on whether to consider her "canon" or not.
- Star-Crossed Lovers: With Pietro, a local mafia's White Sheep.
- Team Mom: She is fiercely protective of the other subjects in the McCallum Children's Home, and taught Calypso to read.
- Too Much Alike: She and Max do not get along because they're both stubborn and distrustful.
A mathematical prodigy whose autism gives him speech difficulties. One of the "lab rats" living in the McCallum Children's Home.
- The Cassandra: He told Hawk for years that he'd developed a hack that could depose McCallum, but she never took him seriously.
- Dying Moment of Awesome: He was impaled right after initiating a code that shut down everyone's biometric guns, enabling a rebellion that frees the City of the Dead for good.
- Hollywood Autism: His entire character is "has autism". He doesn't have interest in anything but his stereotypically STEM-based hyperfixation, or any preferences unrelated to his disorder, or autonomy outside of following the neurotypicals in his life around and helping them with whatever they choose to do. In Real Life, autistic people have as diverse personalities as the rest of humanity, and all but the lowest-functioning (which Clete is not) have hobbies and goals of their own.
- In Chapter 87, Clete calmly, logically explains that he wants to keep his routine. Hawk helps him do so, later explaining to strangers that "if you made him stay here one minute longer, you wouldve seen a meltdown big enough to pollute Tetras air as badly as the City of the Dead." Meltdowns do not work that way. They are rapid changes, but they are always preceded by liminal signs like the person retreating, going nonverbal, getting frustrated, etc, that are not hard for experienced people to notice. An autistic person who can politely vocalize their problem is a long way from degrading into a screaming fit- they are not toddlers. Even then, someone's maturity and level of functioning makes a big difference in how intensely they meltdown. A fifteen-year-old who's high-functioning enough to survive in a Wretched Hive, even while throwing a fit, will not yell or lash out as would a low-functioning seven-year-old who can't dress themselves.
An insecure girl whose exposure to acid rain warped her skin. One of the "lab rats" living in the McCallum Children's Home.
An eight-year-old with insectoid features. One of the "lab rats" living in the McCallum Children's Home.
- Super Strength: She can lift almost as much weight as Moke, a boy twice her size and age.
A young adult whose body is distinctly bluish, because of scientists' attempts to meld it with silver. One of the "lab rats" living in the McCallum Children's Home.
- Hypocrite: He calls Clete a freak, despite being abnormal himself. Hawk lampshades how stupid it is.
The heir of Giacomo Pater, an abusive ganglord. He is the complete opposite of his father- a peaceful and egalitarian teenager whose infatuation with Hawk frequently gets him into trouble.
- Dogged Nice Guy: Hawk is attracted to Pietro, but he has much stronger feelings for her than she does for him.
Hawk's hawk (yes) companion, who she considers a friend more than a pet.
A puppy of Total's who inherited his wings.